The Oscar-winning actress sent the message to four-year-old Will after learning that he was a big fan of "Legally Blonde."

"This is for one of the strongest little four-year-olds I can imagine. Will - I hear that you're fighting so hard and strong against cancer and that in the hospital you've been watching a lot of Legally Blonde and loving it. I hope it makes you laugh!" Witherspoon captioned an Instagram photo of herself holding a sign with the hashtag "Team Will."

"I wanted you to know that I made this sign in your honor - and I'm sending a big big hug and my hope and prayers. Just know - I'm a really big fan of YOURS!" Witherspoon wrote. "PS anyone else - feel free to make your own sign for Will and post it with the hashtag #TeamWill so he can see it….I'm sure he'd love that!"

In addition, the mother of three made a short YouTube video titled, "The Pink Lady's Message To Will."